This invention relates to polycrystalline diamond inserts for use in rolling cone earth-boring bits. Specifically, this invention relates to tungsten carbide inserts with a diamond cap and which have multiple layers within the carbide body that vary in mechanical properties to reduce residual stress at the interface between the diamond cap and the carbide body.
Earth-boring bits of the type concerned herein have a body with at least one bearing pin. A rolling cone rotatably mounts to the bearing pin. Some cones use teeth integrally formed in the metal of the cone. Others use tungsten carbide inserts pressed into mating holes in the cone. Each insert has a cutting end that protrudes from the hole for engaging the earth formation.
Originally, the inserts were formed entirely of sintered tungsten carbide. In more recent years, however, some have been capped with a diamond layer. The diamond layer is typically formed on the carbide body in a high temperature-high pressure (HTHP) sintering process. In the process, polycrystalline diamond (xe2x80x9cPCDxe2x80x9d) powder is placed in a refractory container. A pre-sintered carbide body is inserted into the container. Then high pressure and high temperature are applied to sinter the PCD to the carbide body. It is known that PCD layers inherently have residual stresses at the interface between the tungsten carbide material and the polycrystalline diamond material. The carbide material, being already sintered, shrinks very little in the HTHP process, while the diamond material will shrink during the process. There is a substantial mismatch of the coefficient of thermal expansion of the PCD layer and the carbide support as the part is cooled down from the HTHP apparatus. The difference in shrinkage results in stress at the interface between the PCD layer and the tungsten carbide body. Fracturing of the PCD layer can result, often occurring at the interface between the PCD layer and the carbide body. This can result in delamination under the extreme temperatures and forces of drilling.
Various solutions have been suggested in the art for modifying the residual stresses existing between a diamond layer and tungsten carbide body. In one technique, the interface geometry is reconfigured to redistribute the stresses. A variety of interface configurations have been disclosed and used.
In this invention, an insert is provided for an earth-boring bit of the type having a rolling cone. The inserts are pressed into mating holes in the cone. Each insert has a cutting end that protrudes from the hole in the cone for engaging the earth formation. Each of the inserts has a cylindrical base that locates within one of the holes and a convex end that protrudes from the hole. A polycrystalline diamond cap is bonded to the convex end.
The body is formed of a plurality of elements or layers of carbide material. Each of the layers is free of diamond material, but differs from the other layers in mechanical properties, particularly in the modulus of elasticity and the coefficient of thermal expansion (CTE). The differences are selected to reduce stress at the interface between the convex end and the diamond cap. A higher modulus of elasticity, which is harder and less elastic, is adjacent the diamond layer for providing highly stable support. The layers spaced from the diamond layer have a lesser modulus of elasticity for avoiding excessive brittleness and providing toughness. Also, the CTE of the carbide layer adjacent the diamond layer would be lower than the next adjacent layer.
The different mechanical properties may be achieved by at least the following three different methods: (1) varying the percentage of binder in the carbide; (2) varying the average grain size of the carbide in the carbide layer; or (3) varying the binders from one material to another material. Normally, performing any one of the three methods will result in not only a change in modulus of elasticity but also a change in CTE. Combinations of these three methods may also be made.
In the preferred embodiment, each layer has a different percentage of binder material relative to the carbide material. Preferably the layer with the lowest percentage of binder material is bonded directly to the PCD layer, this layer having the highest modulus of elasticity and the lowest CTE. The layer with the highest percentage of binder material is farthest from the PCD layer, this layer having the lowest modulus of elasticity and the highest CTE. If the average grain size of the carbide material is varied, the carbide material in the layer next to the diamond layer may be of smaller dimension than the average grain size of the other layers. If the binder material itself is varied, some of the layers may contain nickel as the binder, or nickel alloyed with cobalt. The layer with the most cobalt content should be adjacent the PCD layer.